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Study of the sorption hysteresis loop mechanisms in micropore materials
Title: Study of the sorption hysteresis loop mechanisms in micropore materials
Introduction
Hysteresis loop appears when the adsorption and desorption curves do not overlap in sorption isotherms. Research by now shows that all kinds of porous materials can exhibit hysteresis loop regardless of the pore size. For instance, when the pore diameter of a typical mesoporous activated carbon is far larger than the kinetic diameter of the gas molecule, the hysteresis loop appears when the phase transition occurs, resulting in the difficulty of effusion during desorption. Besides, hysteresis loop also occurs in other situations in microporous materials such as metal-organic frameworks （MOFs）and zeolites where the pore apertures are in the same order of the kinetic diameters of gas molecules. This kind of hysteresis has been confirmed to be different from what state above, and is relevant to transition of the framework and/or differences in kinetic energy barriers between adsorption and desorption. Therefore，fundamentally understanding the mechanisms of sorption hysteresis on micropore materials is critical to design and exploit efficient porous materials for adsorbing specific kinds of gases.
Usually, the hysteresis in microporous materials can be classified into two categories- with substantial framework structural change and without such change. For flexible materials such as MOFs, hysteresis is usually affected by the type of gas molecules, temperature and pressure, etc. Hysteresis loops usually occur accompanying with transition of the framework, and such change includes the reversible expansion, distortion1 and rotation of the constituents2. But it should also be pointed out that the transition of framework does not necessarily cause hysteresis loop3, and the hysteresis without structural change will be discussed in the paragraph after “3”.
As discussed above, sorption conditions (gas type, T and P) have strong influences on hysteresis for flexible materials. But for rigid materials even some flexible materials4, there is no substantial structural change when hysteresis occurs. In such cases, the characteristics of metal cations and pore size in zeolites or metal oxides play important roles in deciding the hysteresis mechanisms once the structure does not change.
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Students are more influenced by teachers than by their friends.
With the quicking pace of the morden life and the increasing competitive pressure to the social lives, there are more and more debates about the teenagers' education. Someone consider that students are more influenced by teachers, but some others states that they are more influenced by their friends that played with them. As for this point, my opinion is that students are more likely to be effected by their teachers.
First, students spend much more time with their teachers than play with their friends. As we all know that, the duty of students is to study and work hard, so they spend much of the daytime in the classroom. They have various courses to take, and each course require them to focus on the teacher and listen to what the teacher said. Take myself as an example, when I was young, I did not like to eat vegetables, when taken meals, I just ate what I like and what I want to eat. My parents tried to persuade me to obtain various nutrients, but I just did not listen. But one day, on a biology class, the teacher taught us knowladge about our body and the food we need to eat, she also told us that we should eat plenty kinds of food and should not eat what we just liked, or we wii not grow up healthily. I was convienced and after that class, I did not choose food any more, and ate various of vegetables. Even my parents were surprised.
Second, as teachers are adults, their words are more effective and convencing than children. When are more likely to trust adults than children, besause they are much older than us,they know more knowledge and have more experience. For example, once when I was a child, I played with my friends, and suddenly we saw a beautiful rainbow, we did not know how it generated.Jack, one of my friends said that because someone flied into the sky and painted it. But that reason could not make us believe him, because we did not see any one else in the sky. Until we were taught physics did we know that it was because the water balls and the light reflecting that caused the rainbow.
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